The authors found that the feeling of authorship for mental actions such as solving problems is enhanced by effort cues experienced during mental activity; misattribution of effort cues resulted in inadvertent plagiarism. Pairs of participants took turns solving anagrams as they exerted effort on an unrelated task. People inadvertently plagiarized their partners’ answers more often when they experienced high incidental effort while working on the problem and reduced effort as the solution appeared. This result was found for efforts produced when participants squeezed a handgrip during the task (Experiment 1) or when the anagram was displayed in a font that was difficult to read (Experiments 2, 3a, and 3b). Plagiarism declined, however, when participants attended to the source of the effort cues (Experiments 3a and 3b). These results suggest that effort misattribution can influence authorship processing for mental activities.